1. Field of the Invention
The present invention relates to a resist polymer used for the production of semiconductors and a method for the production thereof. More particularly, the present invention relates to the resist polymer, which is suitable for microfabrication using various radiations such as far ultraviolet ray, X-ray and electron beam, and excellent in storage stability, and the method for production thereof.
2. Description of Related Art
In semiconductor lithography, the formation of finer patterns has been required in conjunction with increase of integration degree. It is, essential for micropatterning techniques to make a wavelength of light source shorter. Currently, the lithography by krypton fluoride (KrF) excimer laser (wavelength 248 nm) has become a mainstream, and the micropatterning technique of 100 nm linewidth or less by argon fluoride (ArF) excimer laser (wavelength 193 nm) is coming into practical use. The micropatterning techniques using fluorine dimer (F2) excimer laser (wavelength 157 nm), extreme-ultraviolet ray (EUV), X-ray, electron beam, and the like are in developmental stages.
The resist polymer used for these lithography technologies has essentially a repeating unit having a non-polar substitute which is decomposed by an acid and becomes soluble in an alkali developer and a repeating unit having a polar group to enhance adhesion to a semiconductor substrate. And if it is necessary, the resist polymer is comprising a repeating unit having non-polar substitutes to regulate solubility in a resist solvent or the alkali developer. As these repeating units, for example, in the KrF lithography, hydroxystyrenes and derivatives thereof have been primarily used, and in the ArF lithography, (meth)acrylates and derivatives-thereof and the like have been considered because the hydroxystyrenes absorb the light with a wavelength of 193 nm.
As specific examples of such resist polymers, copolymers of (meth) acrylic monomer with styrene type monomer (see e.g., Patent References 1 to 4), polymers containing hydroxystyrene which is partially protected with acetal (see e.g., Patent References 5 to 8) and the like are known in the KrF lithography, and copolymers of (meth) acrylic monomer having lactone structure (see e.g., Patent references 9 to 10) are known in the ArF lithography.
Meanwhile, as the resist pattern becomes finer, high quality of the resist polymer is strictly required. In particular, the method for stable production of the resist polymer which is small in lot-to-lot, reactor-to-reactor and scale-to-scale variations is strongly required. As the commonest method for producing the resist polymer, a so-called batch polymerization method has been known in which basic ingredient monomers, a polymerization initiator and if necessary a chain transfer agent are dissolved in a polymerization solvent and polymerized with heat. However, in the batch polymerization method, it is difficult to control a temperature-rising rate constantly at all times. Therefore, there has been a drawback that difference in amount of generated radical species occurs at a stage of temperature-rising due to subtle variations in the temperature-rising rate and causes lot-to-lot variations in molecular weight distribution. Also, it has been necessary to consider and change a heating condition in order to reproduce the same temperature-rising rate regardless of shape and scale of the reactor. In addition, in the case of batch polymerization method, even if the same temperature-rising rate is reproduced, there has been a drawback that difference of a radical concentration occurs in the temperature-rising process due to batch-to-batch variations in amount of dissolved oxygen, because a trace quantity of radical generated by partial decomposition of the polymerization initiator in the temperature-rising process is trapped by dissolved oxygen in the polymerization solvent. And thus difference easily occurs in weight average molecular weight and molecular weight distribution. Additionally, since there is a period of a low radical concentration and a high monomer concentration in the temperature-rising process in the batch polymerization method, there has been a problem that a trace amount of a high molecular weight component (high polymer) with molecular weight of 100,000 or more generates.
As the polymerization method which is not subject to the temperature-rising rate, the method in which monomers and a chain transfer agent as necessary are dissolved in the polymerization solvent and heated to the polymerization temperature followed by adding a polymerization initiator at a time or in some portions (see e.g., Patent Reference 11) and the method in which the polymerization initiator is added by dropwise over the definite time (see e.g., Patent Reference 12) are known. However, there has been a problem that a trace amount of high polymer generates because the monomer is heated to the polymerization temperature in the absence of the polymerization initiator in these methods.
Also, the method, so-called dropping polymerization method is known, in which the polymerization is carried out by dropping a solution containing monomers, a polymerization initiator and if necessary a chain transfer reagent in the same solvent as the polymerization solvent or different, into the polymerization solvent heated up to the polymerization temperature (see e.g., Patent References 13 to 15). However, there has been a problem that a trace amount of high polymer generates because the monomer is retained in the coexistence with a polymerization initiator during the period of the dropwise addition in this method.
As the method for suppressing the generation of high polymer in a mixed solution of a monomer and a polymerization initiator, the method in which a polymerization inhibitor is added is considered. However, when a large amount of polymerization inhibitor is added to suppress the generation of high polymer, it likely affects the polymerization reaction. In addition, in the ArF lithography, there has been a problem that a phenol type compound such as 4-methoxyphenol generally used as the polymerization inhibitor is left in the polymer and absorbs the light with a wavelength of 193 nm. Besides, according to the study by the present inventors, radical species can be consumed by dissolved oxygen and the generation of high polymer can be suppressed to some extent, by retaining the mixed solution of the monomer and the polymerization initiator under an air atmosphere instead of adding the polymerization inhibitor. But there is a possibility of explosion and fire disaster in this method, which could not be industrially practiced in terms of safety.
When the above high polymer generates, solubility of the obtained resist polymer to a resist solution and an alkali developer becomes poor. Even if the high polymer is dissolved initially, it nucleates and sometimes makes an insoluble foreign substance grow and precipitate in storage with time. It is highly possible that these insoluble matters become a cause of defects of the resist pattern. Thus, it has been strongly desired to provide a resist polymer which does not contain the high polymer and a method for the production thereof.
Patent Reference 1                JP-A-59-45439        
Patent Reference 2                JP-A-5-113667        
Patent Reference 3                JP-A-7-209868        
Patent Reference 4                JP-A-11-65120        
Patent Reference 5                JP-A-62-115440.        
Patent Reference 6                JP-A-4-219757        
Patent Reference 7                JP-A-3-223860        
Patent Reference 8                JP-A-4-104251        
Patent Reference 9                JP-A-9-73173        
Patent Reference 10                JP-A-10-239846        
Patent Reference 11                JP-A-2001-109153        
Patent Reference 12                JP-A-2002-6501 (Examples)        
Patent Reference 13                JP-A-4-269754 (Examples)        
Patent Reference 14                JP-A-11-295894        
Patent Reference 15                International Publication WO99/50322        